While the application of microwave radiation to fracture relatively soft manmade surfaces such as concrete is recognized by the prior art, to date no one has applied that technology to fracture naturally occurring hard rock formations in a mining environment.
During the past twenty years on cutting and fragmentation of hard rock, it was realized that the metallurgical strength limits had been reached with conventional mechanical systems. An alternative way to make improvements in cutting and fragmenting of hard rock is to preweaken/fracture it ahead of the mechanical cutting tool by applying another form of energy. Past and current research has shown microwave energy to be a viable candidate for a combination energy/cutting fragmentation system. The selection of a suitable method for fragmentation is based, among other factors, on economic and practical operating requirements. Modifying and improving existing methods and developing new methods of fragmentation becomes necessary for cost reduction and increasing the speed and efficiency of operation.
A wide variety of mechanical fragmentation machinery, such as boring, tunneling, and continuous mining machines, are available for cutting rock formations having strengths ranging from soft up to the lower range of medium hard (12-&gt;25 Kpsi compressive strength). However, for formations in the upper ranges of medium-hard to hard rock (&gt;25 Kpsi) this type of machinery will not be able to cope competitively.
The physics and mechanics of rock fragmentation employing mechanical tools is well understood. The problem is two-fold: first, the inability of many excavators to provide the high thrust and torque necessary to achieve acceptable production rates, and/or second, the inability of the mechanical cutters to survive the high forces encountered in hard rock cutting. Current indications are that the tungsten carbide, used as the bit cutting surface, has been taken to its limit and further improvements are not expected. Therefore, the drag bits used as the mechanical tools have likewise reached their limits.
Previous research on thermally-assisted cutting of hard rock employing surface heating techniques showed the heat-weakening concept technically feasible, but economically and practically unattractive for gas jets, lasers and radiant electric heaters. Subsurface fracturing and weakening of the rock was achieved, but is limited to a slow rate due to the thermal properties of the material.
Previous patents on microwave fracturing of concrete and other brittle materials have used the microwave energy alone to fragment the material. All operate on the principal of differential thermal expansion causing tensile stress fracturing to occur within the material. The combined process of microwave-mechanical cutting is not mentioned.
Rock fragmentation is a basic requirement of the minerals industry. The term "fragmentation" is often associated with irreversible structural changes in the failure of crystalline solids and is defined here as the process of breaking a rock into two or more parts by separation and formation of new surfaces. This physical irreversibility results from energy dissipation within the rock. Rock is herein defined as a polycrystalline aggregate or amorphous solid composed of one or more minerals in aggregate and includes the categories, basalt, granite, gabbro, multiphase ore, and quartzites. Hard rock is herein defined as the rock above having a confined compressive strength greater than 25,000 psi.